Transcript File - SOUTHERN MEDICAL UNIVERSITY

Immunoglobulin
Structure and Function
Immunoglobulins: Structure and
Function
• Definition: Glycoprotein molecules that are
produced by plasma cells in response to an
immunogen and which function as antibodies
Amount of protein
+
albumin
globulins
α1
α2
β
γ
Immune serum
Ag adsorbed serum
Mobility
ANTIBODIES
• Immunoglobulins : Proteins of animal origin endowed with
antibody activity
• Immune sera
• Gamma globulins
• All antibodies are immunoglobulins, but
all immunoglobulins may not be antibodies
• Immunoglobulins : Structural & Chemical concept
• Antibodies
: Functional concept
• Immunogloblins : 20 – 25 % of total serum proteins
• Five classes of Immunoglobulins : IgG, IgA, IM, IgD, IgE
STRUCTURE
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Cleavage study : Fc and Fab fractions
Glycoproteins consisting of 2 pairs of polypeptide chains
L chains (Light) & H chains (Heavy)
L chains : Mol wt 25,000, Attached to heavy chains by a
disulphide bond
- Occur in 2 varieties : 1. Kappa (Ќ)
2. Lambda (λ)
• H chains : Mol wt 50,000, Two h chains joined together
by 1-5 SS bonds
- 5 classes of H chains : ‫ ץ‬α μ δ ε
Immunoglobulin Fragments:
Structure/Function Relationships
Ag Binding
Complement Binding Site
Binding to Fc
Receptors
Placental Transfer
Immunoglobulin Fragments:
Structure/Function Relationships
• Fab
Papain
– Ag binding
– Valence = 1
– Specificity
determined by VH
and VL
• Fc
Fc
– Effector functions
Fab
Immunoglobulin Fragments:
Structure/Function Relationships
• Fab
Pepsin
– Ag binding
• Fc
– Effector functions
• F(ab’)2
Fc
Peptides
F(ab’)2
• Antigen combing site : Aminoterminus
• Fc fragment site : Carboxy terminal for other biological activities
- Complement fixation
- Placental transfer
- Skin fixation & catabolic rate
• Variable and Constant regions
• Hypervariable regions (Hot spots) : CDR’s (Complementarity
determining regions)
• Fd piece : portion of H chain in Fab fragment
• Globular domains : Inter chain disulphide bonds form loops in the
peptide chain and lops are compactly folded, having specific functions
- Vl and VH for formation of specific antigen binding site
- CH2 binds C1q complement
- CH3 mediates adherence to monocyte surface
• Hinge region : Flexible exposed to enzymes an chemicals
Structure of the Variable Region
• Hypervariable (HVR) or complimentarity
determining regions (CDR)
Variability Index
HVR3
150
100
HVR2
HVR1
50
FR2
FR1
0
25
FR3
75
50
Amino acid residue
• Framework regions
FR4
100
Human Immunoglobulin Classes
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IgG - Gamma (γ) heavy chains
IgA - Alpha (α) heavy chains
IgM - Mu (µ) heavy chains
IgD - Delta (δ) heavy chains
IgE - Epsilon (ε) heavy chains
Human Immunoglobulin
Subclasses
• IgG Subclasses
– IgG1 - Gamma 1 (γ1) heavy chains
– IgG2 - Gamma 2 (γ2) heavy chains
– IgG3 - Gamma 3 (γ3) heavy chains
– IgG4 - Gamma 4 (γ4) heavy chains
• IgA subclasses
– IgA1 - Alpha 1 (α1) heavy chains
– IgA2 - Alpha 2 (α2) heavy chains
+
IgG
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Major serum immunoglobulin – 80%
Molecular wt 1,50,000 (7S) , occasionally in polymersed form
Distributed equally between intravascular an extra vascular
Half life : 23 days
Serum concentration : 8 to 16 mg per ml
Transported across placenta
Binds to microoranisms and enhances phagocytosis
General purpose antibody active in blood and tissues
Participates in Complement fixation, Precipitation and
neutralization of toxins and viruses
• Four sub classes : IgG1, IgG2, IgG3, IgG4
- Serum conc : 65%, 23%, 8%, 4% resp
IgG
• Structure
– Monomer (7S)
IgG1, IgG2 and IgG4
IgG3
IgA
• Structure
– Serum - monomer
– Secretions (sIgA)
• Dimer (11S)
• J chain
• Secretory
component
Secretory Piece
J Chain
IgA
• 10 to 13% of serum immunoglobulin, 0.6 to 4.2 mg/ml
• Major Ig in colostrum, saliva and tears, Half life 6-8 days
• 1. Serum IgA – Monomeric 7S (Mol wt 1,60,000)
2. Secretory IgA : Dimeric SIgA , 11 S (4,00,00 mol wt)
J chain : IgA found on mucosal surfaces and in secretions
is a dimer joined by 2 monomer units joined together at
carboxy terminals by a glycoprotein
Secretory component : Glycine rich polypeptide produced by
mucosal cells, protect IgA from denaturation by bacterial
proteases
• Antibody paste, inhibit adherence of microorganisms to
mucosal surface, promotes phagocytosis, activate alternate
Complement pathway
• Two sub class : IgA1 & IgA2
Origin of Secretory Component of sIgA
IgM
• Structure
– Pentamer (19S)
– Extra domain
(CH4)
– J chain
J Chain
Cµ4
IgM
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5- 8% of serum Ig, 0.5 – 2mg/ml, Half life 5 days
Millionaire molecule : 19 S, 9,00,000 mol wt
Polymers of 5 subunits with J chain
Oldest and earliest, short lived Ig, 80% intravascular
Produced by foetus at 20 weeks of age
Iso haemagglutinins, Natural abs to micro organisms, Abs to
Typhoid O Ag, Reagin Abs in syphilis
• Monomeric IgM is the major antibody receptor on surface of
B lymphocytes for antigen recognition
• A single IgM molecule bring about Immune haemolysis,
thousand times more effective in opsonisation,100 times
effective in bactericidal action and 20 times in agglutination
B Cell Antigen Receptor (BcR)
Ig-α Ig-β
Ig-α Ig-β
IgD
• Structure
– Monomer
– Tail piece
Tail Piece
IgD
• Resembles IgG structurally
• 3 mg / 100 ml conc in serum
• Mostly intravascular
• Half life 3 days
• Occur on the surface of B lymphocytes and serve as
recognition receptors for antigens
IgE
• Structure
– Monomer
– Extra domain
(CH4)
Cε4
IgE
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Atopic reagin antibodies, Ishizaka 1966
8 S molecule, Molecular wt 1,90,000, Half life 2 days
Extravascular in distribution
Elevated levels in atopic conditions like, asthma, hay
fever, eczema and parasitic infections
• Chiefly produced in linings of respiratory and intestinal
tracts
• Responsible for Anaphylactic type of hypersensitivity
• Protect against pathogens by mast cell de-granulations
and release of inflammatory mediators
ABNORMAL IMMUNOGLOBULINS
• Structurally similar proteins to antibodies in serum
• Seen in many pathological processes
• Bence Jones Proteins in Multiple myeloma : Light chains of
immunoglobulins, either kappa or lambda
- Identified in urine by coagulation when heated to 50 0C but
redissolving at 70 0 C
- Multiple myeloma may affect Ig G,A,D,E
• Waldenstrom’s macrogluobulinemia : Myeloma of IgM
producing plasma cells – Excessive production of M proteins
• Cryoglobulinemia : Formation of a gel or precipitate on cooling
the serum, which redissolves on warming
- Associated with myelomas, macroglobulinaemias and
autoimmune conditions such as systemic lupus erythematosus
Immunoglobulin specificities
• Idiotopes : Specific antigenic determinants
on paratope
• Idiotype : Sum total of idiotopes on Ig
molecule
• Antiidiotypic antibodies : Produced by
immunization with Fab fragments,
resembles epitopes of original antigen
• Used as vaccines to protect against
pathogen or tumour
Isotypic and Allotypic specificities
• Genetically determined specificities based on their antigenic
structure
• Isotypic specificity : Antigenic specificity which distinguish
between the different classes and subclasses of Ig’s present in all
normal individuals of a given species
• Allotypic specificity : Antigenic specificity which distinguish Ig’s of
the same class between different groups of individuals in the same
species
• Anti-allotype specific abs may develop following blood transfusion or
passage of maternal IgG into foetus and also seen in sera
containing RA factor.
• 2 Allotype systems in humans : 1. Gm system 2. InV system
• Gm is associated with Fc portion of IgG heavy chain, more than 25
Gm types identified so far
• InV associated with kappa light chain (Km) and 3 Km types
• Am : Genetic markers associated with IgA